Classical screening methods for anticancer drugs have been based on their activities to kill cancer cells or to induce shrinkage of tumor grafts. The drugs selected by these methods, however, generally have serious side effects and narrow ranges of tolerated doses. Recently molecular-targeted drugs have been spotlighted, but in most cases they are applicable only to tumors bearing genetic alterations in a specific gene and a second mutation could develop resistant cells. Therefore there is a pressing need to discover drugs with few side effects and strong anticancer activities.
The inventors have established an original screening system for malignant transformation suppressor genes and using this system found out novel cancer-associated genes. It has been revealed that, among the novel cancer-associated genes that the inventors have found out, the RECK (reversion-inducing-cysteine-rich protein with kazal motifs) gene encoding a membrane-anchored metalloproteinase-regulator is often down-regulated in various types of cancers such as large intestine, lung, stomach, breast, pancreas, and prostate cancers and the expression of the RECK gene in tumor tissues positively correlates with the survival of the patients. In addition, transplantation of a cancer cell line with forced expression of the RECK gene into nude mice resulted in more suppression of tumor proliferation, angiogenesis, invasion, metastasis, and the like as compared to transplantation of the parent strain. These findings imply that RECK is not only a useful prognosis marker but also a promising target molecule to be activated (effector) in cancer therapy (Non Patent Literature 1, 2 and 3).
In cancer metastasis experiments, the most commonly used assay is a method in which two to eight weeks after injection of melanoma cells into the tail veins of mice, the mice are dissected and the number of the colonies developed in the lungs is counted (tail vein assay) (Non Patent Literature 4 and 5). This assay is also referred to as “experimental metastasis assay” and considered to be the reproduction of the later stage of hematogenous metastasis. Another method in which injection of tumor cells into a particular tissue results in the development of tumors at distant tissues is more close to metastasis in actual patients and is referred to as “spontaneous metastasis assay”. One of known spontaneous metastasis protocols is that tumors are inoculated into the foot-pad of mice; once the tumors reach a predetermined size, the tumor-bearing foot is amputated for removal of the primary tumors; and 40 to 100 days later the mice are dissected for examination of lung metastasis (Non Patent Literature 6). These assays, however, require a long time for the results to show up and therefore there is a need to establish an experimental system to evaluate spontaneous metastasis in a short period of time.